Osteoarthritis, the commonest form of arthritis, is a disease of cartilage. Cartilage is composed primarily of proteoglycans- core protein with side chains of glycosaminoglycans (mucopolysaccharides)- and collagen. Not a great deal is known about the mechanism of cartilage production by chondrocytes or the regulation of its synthesis. Cartilage can be studied in vitro in cell cultures of chondrocytes or organ cultures of cartilage fragments. The type of proteoglycan synthesized under a variety of conditions is being studied. These studies should provide information useful to understanding the dynamics of cartilage changes in osteoarthritis.